In the manufacture of multilevel interconnects for integrated circuits, aluminum contacts have been deposited over nitrogen annealed titanium layers for several reasons. A titanium layer can be sputter deposited directly over bare silicon or polysilicon surfaces on integrated circuit substrates, and annealed at elevated temperatures. During the annealing step, the titanium reacts with the underlying silicon to form titanium silicide (TiSi.sub.x), a low resistance material; when the titanium layer is annealed in the presence of nitrogen, it also reacts with the nitrogen to form a surface titanium nitride layer that acts as a barrier to an overlying aluminum contact. This TiN barrier layer prevents the aluminum from spiking through to the underlying silicon, which would short out devices in the silicon, and prevents the silicon and any dopants that are present in the semiconductor substrate from diffusing to the aluminum contact.
In an effort to improve the Ti/TiN barrier properties, a first layer of titanium has been deposited and a second layer of titanium nitride has been deposited thereover. The titanium-containing layers are then heated or annealed at high temperatures under nitrogen to form the TiSi.sub.x layer. The presence of nitrogen prevents the formation of oxide on the titanium nitride layer.
As the number of integrated circuit devices on a single silicon wafer has become larger, the devices themselves have become smaller and more densely packed, and the processing of individual devices becomes more critical. When aluminum is deposited into high aspect ratio openings or trenches in silicon, higher temperatures of deposition must be used to fill the trenches completely and to form a planarized surface. However, when high aluminum deposition temperatures are used, the prior art titanium nitride barrier layers are inadequate to prevent aluminum spiking through the titanium nitride layer.
Thus a means of improving aluminum contacts to provide. stable, low resistance contacts to the underlying silicon substrate, to provide improved barrier properties in titanium-containing barrier layers, and to provide good flow and planarizing properties for overlying aluminum layers has been sought.